1. Field of the Invention
This invention relates to the field of transformers, and in particular to transformers used to provide high voltage for lamps.
2. Description of Related Art
FIG. 1 illustrates an example circuit diagram for an electronic ballast 100, as might be used to provide a high-voltage, high-frequency signal for a cold cathode fluorescent lamp 110 from a relatively low-voltage DC supply signal Vin 120. The controller 130 and switching devices 131, 132 control the current flow through the transformers 140, 150, so as to provide the required current to the lamp 110 during the different stages (ignition, steady state, etc.) of the lamp's illumination.
The high-voltage transformation is provided by the step-up transformer 150, which typically has a high turns ratio, such as 300:1, between the secondary 150s and primary 150p coils. The required high-frequency signal is provided by an LC resonant tank, wherein the transformer 140 form the resonant inductor, and the resonant capacitance is formed by shielding parasitic capacitances and the interwinding capacitance of transformer 150. U.S. Pat. No. 5,495,405, "Inverter Circuit for Use with Discharge Tube", issued Feb. 27, 1996 for Fujimura et al, teaches the use of the parasitic capacitance produced in the secondary side of the step up transformer as a component of the resonant circuit, and is incorporated by reference herein.
FIG. 2 illustrates an example embodiment of the step-up transformer 150, as typically employed in a prior art electronic ballast 100. Each of the coils 150s, 150p, and 150a are wound by rotating a hollow core bobbin 250 in a direction 210, while a wire is wound around the bobbin 250 and laid upon sections of the bobbin 250 in the illustrated winding direction 220. The wires may be wrapped around a common segmented bobbin, or wrapped around individual bobbin segments that are subsequently bonded together to form the segmented bobbin. After the appropriate number of turns of wire are laid upon the bobbin 250, the wire ends of each coil 150s, 150p, and 150a are made available for connection to the other components of the electronic ballast 100 of FIG. 1. In order to maintain the appropriate coil phases indicated by the "dot" phase convention of each coil 150s, 150p, 150a of FIG. 1, the ends of each coil are arranged as discussed below and as indicated by the two alternative node assignments of FIG. 2. If the starting (right) end of the secondary coil 150s is assigned to node 2 (and therefore the terminating (left) end of the secondary coil 150s is assigned to node 4), then node 1 must be assigned to the starting (right) end of the primary coil 150p and node 3 to the terminating (left) end, in order for the primary 150p and secondary 150s coils to have the dot-phase relationship indicated in FIG. 1. In like manner, based on the choice of node 2 as the starting end of the secondary coil 150s, node 6 must be assigned to the start (right) end of the auxiliary coil 150a, and node 8 to the terminating (left) end, in order to maintain the proper phase relationship between the auxiliary coil 150a and the secondary coil 150s. Alternatively, as indicated by the parenthesized node assignments, if node 4 is assigned to the start (right) end of the secondary coil 150s, then node 3 must be assigned to the start (right) end of the primary coil 150p, and node 8 must be assigned to the start (right) end of the auxiliary coil 150a. As would be evident to one of ordinary skill in the art, the bobbin rotation direction 210 may be reversed, and the winding direction 220 may be reversed.
Coils 150a, 150p, and 150s are inductively coupled by their proximity and relationship to each other. The inductive coupling may be increased by providing a ferrite core within the center of the bobbin 250 that traverses the length of the transformer 150. Unfortunately, the proximity and relationship of coils 150p and 150s also introduces a capacitive coupling between these coils 150p and 150s. This capacitive coupling adversely affects the circuit operation of the electronic ballast 100, and severely limits the voltage gain of the step-up transformer 150.